The invention will be described in its application to the food industry, but those skilled in the art will appreciate its applicability to other fields such as pharmaceuticals and mining. The food industry already screens its processes for physical contamination. Metallic parts such as nuts or rivets are easy to remove because metal detectors signal their presence. Plastic parts such as mixer blades, or rubber parts such as O-rings sometimes fail and these may produce fragments of different sizes. This problem is dealt with in U.S. Pat. Nos. 6,113,482 and 6,177,113. Incorporation of stainless steel particles in the machinery components which are susceptible to breakage ensures that the fragments elect a signal from a metal detector in the conveyor on which all products must pass. The detectors which routinely screen the product are tuned to find specific sizes and are calibrated with 1 mm diameter test balls. It is unrealistic to rely on these to detect smaller metal contaminants.
In confectionery making by factory processes, a special problem arises. It is known in to cast jelly confections in cavities pressed into powders such as starch. The starch is laid as a bed in shallow trays which pass through a low temperature oven. After the trays of jelly confections have passed through the oven they are inverted over a sieve for reuse. The inversion and other tray handling operations are performed mechanically, and inevitably a percentage of trays break and fragments contaminate the food. The trays are commonly made of plywood, which dries out with repeated passages through the oven, and if the tray shatters the contamination may affect more than one tray of food. Such failures are a problem and much wholesome food must be discarded to ensure that the result of the failure has been eliminated from the food line and the starch line. The starch in the starch line is recycled continuously and it is imperative to ensure that contaminants are not recycled at the same time. The aim is therefore to remove contaminants before they become embedded in food because then they become more difficult to remove.
A change to thermoset plastic trays which are stronger than plywood has not removed the danger but has exchanged one type of contamination for another. Such thermosetting resins are referred to as sheet moulding compositions (SMC).